A known "grey scale" driver circuit comprises an input stage in the form of a level shifter, a capacitor circuit connected to the level shifter and operating as a sample and hold (SH) switch and an output stage in the form of a voltage follower. This circuit receives the following signals:
i) a pulse width modulated logic signal which is supplied to the level shifter and which, when level shifted controls the sampling period of the SH switch; PA1 ii) a ramped analogue voltage which is sampled and held by the SH switch at a time determined by the pulse width modulated logic signal.
The signal which is held by the SH switch is transferred to an output node via the voltage follower which receives a power supply at a high voltage required to supply the necessary potential difference across the electroluminescent material, typically 60V. The high voltage power supply is normally modulated to reduce power consumption in flat panel systems and thus the voltage supplied to the voltage follower will increase and decrease in accordance with the modulation applied to it. The high voltage power supply is normally ramped but is generally out of phase with the ramped analogue voltage supplied to the SH switch. Furthermore, where the voltage follower is made with MOS transistors, these transistors must be large to carry the high voltages required for the display and, as such, have large parasitic capacitances associated therewith. These large parasitic capacitances and the variations in the modulated power supply can cause the circuit to function incorrectly because the output node can become coupled to the signal held at the SH switch through the parasitic capacitance, thereby affecting the output drive voltage from the voltage follower. Where the SH switch is implemented by a capacitor and a transistor switch, the capacitor can be caused to store not only a signal derived from the ramped analogue voltage under the control of the pulse width modulated logic signal but also a signal incorrectly derived from the modulations on the power supply to the voltage follower through the parasitic capacitances associated therewith.
In order to try and overcome this problem, it is conventional to ensure that the capacitor in the SH switch is several times larger than the parasitic capacitances of the voltage follower. However, as the parasitic capacitances of the voltage follower can be substantial, due to the large transistors required, this requires a large capacitor in the SH circuit and a consequential waste of chip area where the circuit is to be made as an integrated circuit on a semiconductor chip.
The present invention seeks to overcome this problem by providing a driver circuit which can utilise a capacitor of a smaller size.